1. Field of the Invention
The present invention relates to steel cords to be used as reinforcements for rubbery articles such as pneumatic tires and industrial belts, and to radial tires having a belt layer composed of such steel cords. The present invention directed particularly to improve durability.
2. Related Art Statement
The rubbery article reinforced with steel cords has a problem in that durability life of the article lowers due to corrosion of steel filaments from water invading it.
For instance, assume that the steel cords used for the belt of the tire have cavities therein. If the tire tread gets an external cut that reaches into the belt, water entering the belt spreads over the cord in a longitudinal direction through the cavity, so that rust formed by water accordingly spread. Consequently, adhesion between the rubber and the steel cord drops at those locations, and a separation phenomenon occurs ultimately.
In order to prevent such a corrosion-propagating phenomenon, a cord construction is proposed, in which rubber can fully penetrate through gaps between adjacent metallic filaments on vulcanizing under pressure.
Japanese Patent Publication Nos. 62-21,641 and 60-49,421 disclose that gaps are provided between filaments of a cord by excessively forming the cord. However, it is difficult to maintain the gaps, so that effects obtained disadvantageously depend upon handling in the tire producing process.
To the contrary, Japanese Utility Model Registration Application Laid-open No. 60-178,204 shows a technique for maintaining gaps between filaments by improving the construction of the cord without relying upon the forming technique. This publication discloses a tire using a belt composed of steel cords each consisting of a core made of two metallic filaments and an outer layer made of six metallic filaments arranged around the core. However, since the cords disclosed are composed of the filaments having a small diameter, it is difficult for such steel cords to assure satisfactory internal pressure-maintaining ability and durability when used for trucks or buses.
That is, it is necessary to increase the number of cords per unit width or the diameter of the cords so as to ensure a sufficient internal pressure-maintaining ability. However, since gaps between the adjacent cords narrow when the number of cords per unit width is increased, belt separation occurs. On the other hand, if the diameter of the cord is increased, strain on the surface of the cord greatly varies on bending, so that buckling fatigue resistance of the cord drops.